1. Field of the Invention
The present invention relates to a cross support for roof loads on a motor vehicle provided with a roof railing, wherein the support extends from one longitudinally extending railing tube to the other and is provided at each end region with a detachable clamping device for attachment to a railing tube. Each clamping device has a resting jaw and a clamping jaw which is movable relative to the resting jaw. A jaw part of the clamping jaw is developed as a yoke which partly surrounds the associated railing tube so as to hold the cross support upon fastening on the clamping device.
2. Description of the Related Art
A cross support of this type is known from Federal Republic of Germany Patent Publication 41 08 058 C1. That known roof luggage carrier includes cooperating resting jaws and clamping jaws provided as fastening means. The resting jaws lie on the side of the respective railing tube that is toward the center of the vehicle and are fastened on the cross support. The clamping jaws lie on the side of the railing tube that is toward the outside of the vehicle and are seated displaceably on the cross support by means of a clamping screw bolt. By tightening the clamping screw bolts, the displaceable clamping jaws are pulled against the railing tubes and toward the resting jaws. This connects the cross support to the railing tubes.
However, it has been found that the clamping devices provided in known cross supports do not satisfy technical requirements. One disadvantage is that the jaws cannot be used for railing tube cross sections of any desired shape but must instead be adapted to the cross section of the specific railing tube. Another disadvantage is that the clamping force obtainable is not always sufficient to dependably hold the cross support on the railing tubes. This is because the unfavorable lever ratio of the clamping screw bolts with respect to the active jaw regions on the railing tubes does not permit optimal transmission of force. Experimentation has shown that, upon tightening of the clamping screw bolts, a relatively large bending moment is exerted on them as a result of the unfavorable lever ratios. This may jam and cant the displaceable clamping jaw. As a result, the clamping force transmitted to the clamping screw bolts is only in part applied as the clamping force acting on the railing tubes. Finally, operation of the clamping devices provided on the known cross supports is extremely cumbersome.